A CRT for generating color pictures generally contains an electron gun emitting three coplanar beams of electrons (R, G and B electron beams), to excite on a screen a luminescent material of a given primary color red, green, and blue, respectively. The deflection yoke is mounted the neck of the tube for producing deflection fields created by the horizontal and vertical deflection coils or windings. A ring or core of ferromagnetic material surrounds, in a conventional way the deflection coils.
The three beams generated are required to converge on the screen for avoiding a beam landing error called convergence error that would otherwise produce an error in the rendering of the colors. In order to provide convergence, it is known to use astigmatic deflection fields called self-converging. In a self-converging deflection coil, the field nonuniformity that is depicted by lines of flux generated by the horizontal deflection coil has generally pincushion shape in a portion of the coil situated in the front part, closer to the screen.
A geometry distortion referred to as pincushion distortion is produced in part because of the non-spherical shape of the screen surface. The distortion of the picture, referred to as North-South at the top and bottom and East-West at the side of the picture, is stronger as the radius of curvature of the screen is greater.
A coma error occurs because the R and B beams, penetrating the deflection zone at a small angle relative to the longitudinal axis of the tube, undergo a supplementary deflection with respect to that of the center G beam. With respect to the horizontal deflection field, coma is generally corrected by producing a barrel shape horizontal deflection field at the beam entrance region or zone of the deflection yoke, behind the aformentioned pincushion field that is used for convergence error correction.
A coma parabola distortion is manifested in a vertical line at the side of the picture by a gradual horizontal direction shift of the green image relative to the mid-point between the red and blue images as the line is followed from the center to the corner of the screen. If the shift is carried out toward the outside or side of the picture, such coma parabola error is conventionally referred to as being positive; if it is carried out toward the inside or center of picture, the coma parabola error is referred to as being negative.
A horizontal trapezium error occurs due to the astigmatism of the field. When the image to be displayed is a rectangular test pattern, this error is manifested on the tube screen by a blue image which, for example, has rotated with respect to the red image, as illustrated in FIG. 6a. The horizontal trapezium error occurs because the conductors making up the horizontal deflection coils, having a winding distribution chosen to optimize other parameters (convergence, geometry, etc.), may produce high-order deflection field coefficients or harmonics that produce trapezium differentials. The trapezium differentials may result in, for example, a slope inversion of the blue image between the point at 1 H (one o'clock on the screen) and the point representing the corner of the image at 2 H (two o'clock on the screen), as illustrated in FIG. 6b.
It is common practice to divide the deflection field into three successive action zones along the longitudinal axis of the tube: the back or rear zone, closest to the electron gun, the intermediate zone and the front zone, closest to the screen. Coma error is corrected by controlling the field in the rear zone. Geometry error is corrected by controlling the field in the front zone. Convergence error is corrected in the rear and intermediate zones and is least affected in the front zone.
For example, in the prior art deflection yoke of FIG. 2, permanent magnets 240, 241, 242 are positioned in front of the deflection yoke to reduce geometry distortions. Other magnets 142 and field shapers are inserted between the horizontal and vertical deflection coils to modify locally the field to reduce coma, parabola coma, and convergence errors.
When the screen has a relatively large radius of curvature greater than 1R, such as 1.5R or more, for example, it becomes more and more difficult to solve the beam landing errors previously described without utilizing magnetic helpers such as shunts or permanent magnets.
It may be desirable reduce error such as the trapezium error, coma parabola error, coma error or convergence error by controlling the winding distributions of the deflection coils without utilizing magnetic helpers such as shunts or permanent magnets.
Eliminating the shunts or permanent magnets is desirable because, disadvantageously, these additional components may produce a heating problem in the yoke related to higher horizontal frequency, particularly when the horizontal frequency is 32 kHz or 64 kHz and more. These additional components may also, undesirably, increase variations among the produced yokes in a manner to degrade the corrections of trapezium, geometry, coma, coma parabola and convergence errors.